Process for the preparation of isovanillin

ABSTRACT

The present invention relates to a novel process for the preparation of isovanillin. 
     The process for the preparation of isovanillin according to the invention is characterized in that it consists in dealkylation being carried out using a strong acid, selectively in the 3-position, on a 3-alkoxy-4-methoxybenzaldehyde wherein said alkoxy group has at least two carbon atoms.

The present invention relates to a novel process for the preparation ofisovanillin.

Isovanillin, or 3-hydroxy-4-methoxybenzaldehyde, is a synthesisintermediate which is used in particular in the pharmaceutical,cosmetics, agrochemical and food fields.

Several routes of access are described in the prior art, and there mayin particular be mentioned processes which consist in starting withpiperonal (or 3,4-(methylenedioxy) benzaldehyde) and in reacting it withsodium methoxide, in the presence of cuprous chloride, in an organicsolvent, namely dimethylformamide [Baratov et al., Zh. Org. Khim., 27,(7), 1578 (1991)]. The main drawbacks of this process are the cost ofthe starting material used and the drawbacks associated with the use ofan organic solvent which must be recovered in order to recycle it.

Isovanillin may also be prepared from veratraldehyde (or3,4-dimethoxybenzaldehyde) by carrying out a selective demethylationusing methionine in methanesulphonic acid [Fujii et al., J. Chem. Soc.Perkin Trans. 1, 20, 2288 (1977)]. The process is long, methanesulphonicacid is expensive and the selectivity for isovanillin in is not verygood.

Kessar et al. [J. Chem. Soc. Chem. Commun., 7, 400 (1983)], alsodescribe a process for obtaining isovanillin by selective O-methylationof protocatechualdehyde (or 3,4-dihyroxybenzaldehyde) by methyl iodide,in the presence of sodium hydride and in dimethyl sulphoxide. Theisovanillin yield obtained is only 65%. This synthetic route alsosuffers from not being economically competitive, since the startingmaterial is not an industrial product and the presence of the organicsolvent increases the production cost.

Another process, which is also expensive, described by Scarpati et al.,[Synth. Commun., 20, (17), 2565 (1990)]consists of the formylation ofguaiacol protected in the form of the acetate, bydichloromethoxymethane, in the presence of titanium tetrachloride indichloromethane, which leads to 3-acetoxy-4-methoxybenzaldehyde which isthen hydrolysed using sodium hydroxide. Besides the large number ofsteps, this process uses dichloromethoxymethane which is an expensiveand toxic product, and also involves an organic solvent.

Moreover, U.S. Pat. No. 3,367,972 describes a process for thepreparation of isovanillin by acid hydrolysis of veratraldehyde.However, the reaction is slow and not very selective. In Example 1, thedegree of conversion of the aldehyde is only 35.8% after 360 min at70.5° -72.3° C., and the isovanillin obtained (33.2%) is accompanied byvanillin (2.9%) which must be separated out.

None of these processes is satisfactory since they are difficult totransfer to the industrial scale, either on account of poor reactionyields or for economic reasons.

The subject of the present invention is to provide an improved processwhich makes it possible to circumvent the abovementioned drawbacks.

It has now been found that isovanillin can be prepared, in a rapidreaction and with a very good reaction yield, according to a processwhich consists in carrying out the dealkylation using a strong acid,selectively in the 3-position, of a 3-alkoxy-4-methoxybenzaldehydewherein said alkoxy group has at least two carbon atoms.

Indeed, it has been demonstrated that isovanillin can be prepared underadvantageous economic conditions insofar as the dealkylation is carriedout on a 3-alkoxy-4-methoxybenzaldehyde whose leaving alkoxy group has acarbon condensation higher than the methoxy group.

According to a preferred embodiment of the invention, the3-alkoxy-4-methoxybenzaldehyde is prepared by O-methylating a3-alkoxy-4-hydroxybenzaldehyde having an alkoxy group which has at leasttwo carbon atoms.

Thus, in its preferred form, the process for the preparation ofisovanillin, according to the invention, comprises:

a first step, in which O-methylation is carried out on a3-alkoxy-4-hydroxybenzaldehyde having an alkoxy group which has at leasttwo carbon atoms, and

a second step, in which the 3-alkoxy-4-methoxybenzaldehyde obtained isselectively dealkylated in the 3-position, with a strong acid, therebyallowing isovanillin to be obtained.

In accordance with the process of the invention, the starting materialused is 3-alkoxy-4-methoxybenzaldehyde corresponding more particularlyto the following formula (I): ##STR1## in the said formula (I), Rrepresents an alkyl or cycloalkyl radical having at least 2 carbonatoms.

The invention preferably involves a 3-alkoxy-4-methoxybenzaldehyde offormula (I) in which the radical R is preferably an alkyl or cycloalkylradical having a number of carbon atoms which ranges between 2 and 8.

Although, from a chemical point of view, the nature of the radicalchosen is of little importance as long as it comprises at least twocarbon atoms, it should be noted that from an economic point of view itis preferable to select the radical R from the ethyl, propyl, isopropyl,n-butyl, isobutyl, sec-butyl and tert-butyl radicals.

Among the substrates of formula (I) most particularly used are3-ethoxy-4-methoxybenzaldehyde, 3-n-propoxy-4-methoxybenzaldehyde,3-isopropoxy-4-methoxybenzaldehyde, 3-n-butoxy-4-methoxybenzaldehyde,3-isobutoxy-4-methoxybenzaldehyde, 3-sec-butoxy-4-methoxybenzaldehydeand 3-tert-butoxy-4-methoxybenzaldehyde.

In accordance with the process of the invention, the3-alkoxy-4-methoxybenzaldehyde is reacted with a strong acid.

Several mandatory conditions govern the choice of the organic solvent. Afirst characteristic feature of this acid is that it is a strong acid.The term strong acid denotes an acid having a pKa in water of less than3.

The lower limit is not of essential nature.

The pKa is defined as the ionic dissociation constant of the acid/basepair when water is used as solvent.

A second characteristic feature of this acid is that it is anon-oxidizing acid under the reaction conditions. Thus, acids such asnitric acid are therefore excluded from the invention.

As examples of acids which are particularly suitable for carrying outthe process of the invention, there may be mentioned halogenated acidssuch as hydrochloric acid, hydrobromic acid and hydriodic acid.

It is also possible to use halogenated or non-halogenated oxyacids suchas sulphuric acid, pyrosulphuric acid halogenated or non-halogenatedsulphonic acids such as fluorosulphonic acid, chlorosulphonic acid,trifluoromethanesulphonic acid, methanesulphonic acid, ethanesulphonicacid, ethanedisulphonic acid, benzenesulphonic acid, benzenedisulphonicacids, toluenesulphonic acids, naphthalenesulphonic acids andnaphthalenedisulphonic acids.

Among these acids, hydrochloric acid, hydrobromic acid and sulphuricacid will preferably be used.

Sulphuric acid is preferably chosen.

The concentration of the starting strong acid may vary according tocommercial availability.

For example, if hydrochloric acid or hydrobromic acid is used, theconcentration of the acid ranges between 30 and 50% by weight.

If sulphuric acid is used, its concentration advantageously rangesbetween 80 and 100% by weight, preferably between 90 and 99% by weight.According to a preferred embodiment of the invention, a sulphuric acidsolution which is as concentrated as possible is used: thisconcentration is between 95 and 99% by weight.

The ratio between the number of moles of strong acid used and the numberof moles of 3-alkoxy-4-methoxybenzaldehyde may vary widely, for examplebetween 2 and 80.

It is preferable for the said molar ratio to be between 3 and 15 andeven more preferably between 4 and 10.

The temperature at which the process of the invention is performed mayvary widely, for example between 0° C. and 150° C.

From an economic point of view, it is advantageous not to heat thereaction medium. However, heating may prove to be necessary in certaincases.

The temperature selected depends on the nature of the acid used and onthe nature of the group R to be removed.

It has been found that the larger and more branched the group R to beremoved, the lower it is possible for the reaction temperature selectedto be.

The appropriate reaction temperature to use may readily be determined bythose skilled in the art by performing simple operations.

Thus, examples illustrating the range of temperatures selected are givenbelow.

When sulphuric acid is used, the temperature generally ranges between 0°C. and 100° C.

When the group R to be removed is a linear alkyl group such as, forexample, ethyl, n-propyl or n-butyl, the reaction temperature chosen isadvantageously between 50° and 90° C., preferably between 60° C. and 80°C.

When R is a branched alkyl group, that is to say a group in which thecarbon atom located in the α-position relative to the oxygen atom is atleast secondary, that is to say an isopropyl, sec-butyl or tert-butylgroup, the reaction temperature is preferably between 0° C. and 40° C.,and even more preferably between 0° C. and 30° C.

If a halogenated acid such as hydrochloric acid or hydrobromic acid isused, the reaction temperature selected, in the case of a linear alkylgroup R, is preferably between 100° and 150° C., and even morepreferably between 100° C. and 130° C.

If the group R is a branched alkyl group, the reaction temperaturechosen is then advantageously between 50° and 100° C., preferablybetween 60° and 80° C.

From a practical point of view, the process of the invention is easy tocarry out since it does not require the use of specific apparatus.

In practice, the process of the invention may be carried out in themanner described below.

The various constituents of the reaction mixture are loaded into thechosen equipment. In a preferred manner, the strong acid is introducedinto the 3-alkoxy-4-methoxybenzaldehyde, but it is also possible tocarry out the inverse operation. Thus, the molten3-alkoxy-4-methoxybenzaldehyde may be introduced into the strong acid orthe substrate may be added in suspension in the strong acid.

It is preferable to perform the process of the invention under anatmosphere of dry gases. An atmosphere of rare gases, preferably argon,may be established, but nitrogen is preferred.

Once the atmosphere of dry gas has been established, the reactionmixture is brought to the chosen temperature, optionally by heating.

The duration of the reaction may vary widely. It is dependent on thenature of the 3-alkoxy-4-methoxybenzaldehyde, the strong acid and theconcentration thereof, and the temperature.

Under the preferred conditions of the invention, which are the use of aconcentrated strong acid, the duration of the reaction varies, forexample, from 30 minutes to 6 hours, according to the temperature atwhich the reaction is carried out.

At the end of the reaction, the reaction mass is returned to roomtemperature. The term room temperature generally refers to a temperatureof between 18° C. and 25° C.

In certain cases, it may turn out to be necessary to cool to atemperature of between 10° C. and 15° C.

One embodiment which makes it possible to separate the desired productfrom the reaction mass is given below, by way of illustration.

Generally, on account of the presence of the concentrated strong acid,the reaction mass is diluted with water or with ice such that theconcentration of strong acid is from 20 to 30% by weight.

The isovanillin formed is extracted with a water-insoluble organicsolvent, and there may in particular be mentioned ether oxides,preferably ethyl ether or isopropyl ether chlorinated hydrocarbons,preferably chloroform or dichloroethane water-insoluble ketones,preferably methyl isobutyl ketone.

Methyl isobutyl ketone is preferably selected.

The organic and aqueous phases are separated.

The aqueous phase contains the strong acid, which may be regenerated andrecycled.

The organic phase is washed with water and a basic agent is then addedsuch that the pH is between 6.5 and 7.0.

Aqueous sodium hydroxide solution is preferably used.

The organic solvent is evaporated off. The isovanillin precipitates. Itis isolated according to the standard techniques of solid/liquidseparation, preferably by filtration.

The solid obtained may be purified, in particular by crystallization.

According to a preferred embodiment of the invention, the3-alkoxy-4-methoxybenzaldehyde is prepared by O-methylating a3-alkoxy-4-hydroxybenzaldehyde having an alkoxy group which has at leasttwo carbon atoms.

In accordance with a preferred embodiment of the process of theinvention, the starting material used is a3-alkoxy-4-hydroxybenzaldehyde corresponding more particularly to thefollowing formula (II): ##STR2## in the said formula (II), R representsan alkyl or cycloalkyl radical having at least 2 carbon atoms.

The invention preferably involves a 3-alkoxy-4-hydroxybenzaldehyde offormula (II) in which the radical R is preferably an alkyl or cycloalkylradical having a number of carbon atoms which may range between 2 and 8.

Among the substrates of formula (II) most particularly used are3-ethoxy-4-hydroxybenzaldehyde, 3-n-propoxy-4-hydroxybenzaldehyde,3-isopropoxy-4-hydroxybenzaldehyde, 3-n-butoxy-4-hydroxybenzaldehyde,3-isobutoxy-4-hydroxybenzaldehyde, 3-sec-butoxy-4-hydroxybenzaldehydeand 3-tert-butoxy-4-hydroxybenzaldehyde.

When 3-ethoxy-4-hydroxybenzaldehyde, which is a commercially availableproduct, is used, the process of the invention is very competitive withrespect to the processes described.

According to the invention, the 3-alkoxy-4-hydroxybenzaldehyde isO-methylated by reacting it with a methylating agent.

Various types of alkylating agent may be used, and there may moreparticularly be mentioned methyl halides, preferably methyl chloride ormethyl bromide, dimethyl sulphate, methyl p-toluenesulphonate and methylmethanesulphonate.

Among the abovementioned alkylating agents, methyl chloride and dimethylsulphate are preferred.

Generally, the O-methylation reaction is advantageously performed at apH of between 8 and 10 and, preferably, above 9.

The abovementioned pH may be obtained, if necessary, by addition of abase, preferably an aqueous solution of an alkali metal hydroxide or ofan alkali metal hydrogen carbonate or carbonate. Any alkali metalhydrogen carbonate, carbonate or hydroxide may be used. The chosenconcentration for the solution of the basic agent is advantageouslyhigh, preferably between 30 and 60% by weight.

However, for economic considerations, sodium hydroxide is preferred andcommercial solutions of sodium hydroxide having a concentration of 30%are more preferably used.

There is no disadvantage in using an organic base, but, from an economicpoint of view, sodium hydroxide is preferably selected.

As regards the amount of the reactants to use in this O-alkylation step,the preferred amounts are defined below.

The concentration of the starting substrate, namely the3-alkoxy-4-hydroxybenzaldehyde, in the reaction medium is advantageouslybetween 10 and 25%.

The amount of alkylating agent employed depends on the amount ofsubstrate to be alkoxylated. It is preferably at least equal to thestoichiometric amount and up to an excess which may be as high as 100%.

In other words, the alkylating agent/3-alkoxy-4-hydroxybenzaldehydemolar ratio ranges between 1.0 and 2.0 and, preferably, between 1.1 and1.5.

As regards the reaction conditions, it is preferable to perform theprocess of the invention under an atmosphere of inert gas. An atmosphereof rare gases, preferably argon, may be established, but it is moreeconomical to use nitrogen.

Once the atmosphere of inert gas has been established, the reactionmixture is brought to the chosen temperature.

The temperature of the O-methylation reaction is not critical; it has aninfluence on the reaction kinetics. Generally, the O-methylationreaction is performed at between 60° C. and 100° C. The reaction ispreferably performed at a temperature of between 80° C. and 95° C.

The duration of the O-methylation reaction depends in particular on thereaction temperature. It usually ranges between 1 hour and 8 hours. Aduration of from 1 to 4 hours is generally sufficient.

According to a practical embodiment of the invention, it is possiblefirstly to introduce the 3-alkoxy-4-hydroxybenzaldehyde, the basicsolution, to bring the mixture to the reaction temperature and then toadd the alkylating agent and optionally the base if this is required inorder to adjust the pH to within the abovementioned region.

At the end of the reaction, after cooling, an organic phase comprisingthe 3-alkoxy-4-methoxybenzaldehyde is recovered.

It is possible to employ the organic phase, without any separation, inthe dealkylation step after having preferably removed the water.

It is also possible to separate out the 3-alkoxy-4-methoxybenzaldehyde,according to the standard separation techniques.

It is possible in particular to separate it by extraction into asuitable, water-insoluble solvent. The solvents mentioned above aresuitable for use, but methyl isobutyl ketone is preferably selected.

The 3-alkoxy-4-methoxybenzaldehyde obtained may optionally be purifiedin a standard manner, by distillation or by crystallization.

It may then be employed in the dealkylation step, according to theprocess of the invention.

The process of the invention makes it possible to obtain isovanillinselectively.

The examples which follow illustrate the invention without, however,limiting the scope thereof.

EXAMPLES

In the examples, the abbreviations have the following meanings:

EVA=ethyl vanillin=3-ethoxy-4-hydroxybenzaldehyde,

EMBA=3-ethoxy-4-methoxybenzaldehyde, ##EQU1##

EXAMPLE 1 Methylation of ethyl vanillin with dimethyl sulphate

After a nitrogen atmosphere has been established, 332.4 g of ethylvanillin are loaded into a 2000 ml 3-necked round-bottomed flask heatedby an electric heating mantle, stirred mechanically, and equipped withtwo dropping funnels, a condenser, devices for measuring the temperatureand the pH, and a nitrogen inlet.

0.5 liter of water is added and the mixture is heated to 90° C.

360 g of aqueous 30.5% sodium hydroxide solution (pH=9) are added,followed by 350 g of dimethyl sulphate, over 2 hours.

The reaction medium is left stirring for 1 hour at 90° C. and is thencooled to 70° C. and allowed to separate by settling.

The upper organic layer is washed and is then distilled at 120° C. undera reduced pressure of 10 mm of mercury (1330 Pa), to give3-ethoxy-4-methoxybenzaldehyde.

The results obtained, after assaying by high performance liquidchromatography, are as follows:

Yield before distillation: 98.4%

Purity before distillation: 97% (3% water)

Purity after distillation: 99.9%

De-ethylation of 3-ethoxy-4-methoxybenzaldehyde with sulphuric acid

After having established an atmosphere of inert gas, 270.5 g of3-ethoxy-4-methoxybenzaldehyde and 742 g of 98% sulphuric acid areloaded into a 1.5 liter jacketed reactor equipped with a mechanicalstirrer, a thermometer and a nitrogen input.

The mixture is heated for 3 hours 30 minutes at 65° C.

The assay by high performance liquid chromatography gives:

Conversion of the EMBA=98.5%

Yield of isovanillin=96%

The reaction mass is cooled and run into 2liters of ice-cold water, withstirring.

The precipitate which forms is dissolved with 2liters of methyl isobutylketone.

The organic phase is separated out; the aqueous phase is reextractedwith 0.75 liter of methyl isobutyl ketone. The mother liquors (30% H₂SO₄) may be regenerated.

The organic phase is washed to pH=7 and is then evaporated at 60° C.under a reduced pressure of about 100 mm of mercury (13300 Pa), to giveisovanillin (214 g) in a purity equal to 95%.

I claim:
 1. Process for the preparation of isovanillin, comprising thesteps of:carrying out a dealkylation, using a strong acid, selectivelyin the 3-position, of a 3-alkoxy-4-methoxybenzaldehyde of the followingformula (I): ##STR3## wherein R represents an alkyl or cycloalkylradical having a number of carbon atoms which ranges between 2 and 8;and recovering the isovanillin.
 2. Process according to claim 1, whereinthe 3-alkoxy-4-methoxybenzaldehyde is 3-ethoxy-4-methoxybenzaldehyde,3-n-propoxy-4-methoxybenzaldehyde, 3-isopropoxy-4-methoxybenzaldehyde,3-n-butoxy-4-methoxybenzaldehyde, 3-isobutoxy-4-methoxybenzaldehyde,3-sec-butoxy-4-methoxybenzaldehyde and3-tert-butoxy-4-methoxybenzaldehyde.
 3. Process according to claim 2,wherein the strong acid has a pKa of less than equal to
 3. 4. Processaccording to claim 3, wherein the strong acid used is chosen from:halogenated acids; halogenated or non-halogenated oxyacids; halogenatedor non-halogenated sulphonic acids and mixtures thereof.
 5. Processaccording to claim 4 wherein the strong acid used is hydrochloric acid,hydrobromic acid or sulphuric acid.
 6. Process according to claim 5,wherein the concentration of strong acid ranges between 30 and 50% byweight when the strong acid is hydrochloric acid or hydrobromic acid. 7.Process according to claim 6, wherein the concentration of strong acidranges between 80 and 100% by weight, when the strong acid used issulphuric acid.
 8. Process according to claim 7, wherein the ratiobetween the number of moles of strong acid used and the number of molesof 3-alkoxy-4-methoxybenzaldehyde ranges between 2 and
 80. 9. Processaccording to claim 8, wherein the reaction temperature ranges between 0°C. and 150° C.
 10. Process according to claim 9, wherein the reactiontemperature ranges: in the case of sulphuric acid, between 50° and 90°C.; and in the case of hydrochloric acid or hydrobromic acid, between100° C. and 150° C.
 11. Process according to claim 10, wherein thesolution of strong acid is introduced into the3-alkoxy-4-methoxybenzaldehyde, or vice versa.
 12. Process according toclaim 11, wherein the molten 3-alkoxy-4-methoxybenzaldehyde isintroduced into the strong acid, or the substrate is added in suspensionin the strong acid.
 13. Process according to claim 12, wherein the3-alkoxy-4-methoxybenzaldehyde is prepared by O-methylating a3-alkoxy-4-hydroxybenzaldehyde having an alkoxy group which has at leasttwo carbon atoms.
 14. Process according to claim 13, wherein the3-alkoxy-4-hydroxybenzaldehyde corresponds to the following formula(II): ##STR4## in the said formula (II), R represents an alkyl orcycloalkyl radical having at least 2 carbon atoms.
 15. Process accordingto claim 14, wherein the 3-alkoxy-4-hydroxybenzaldehyde corresponds tothe formula (II) in which the radical R is an alkyl or cycloalkylradical having a number of carbon atoms which ranges between 2 and 8.16. Process according to claim 15, wherein the3-alkoxy-4-hydroxybenzaldehyde can be 3-ethoxy-4-hydroxybenzaldehyde,3-n-propoxy-4-hydroxybenzaldehyde, 3-isopropoxy-4-hydroxybenzaldehyde,3-n-butoxy-4-hydroxybenzaldehyde, 3-isobutoxy-4-hydroxybenzaldehyde,3-sec-butoxy-4-hydroxybenzaldehyde, and3-tert-butoxy-4-hydroxybenzaldehyde.
 17. Process according to claim 16,wherein the alkylating agent is methylhalide.
 18. Process according toclaim 17, wherein the O-methylation reaction is performed at a pH ofbetween 8 and
 10. 19. Process according to claim 18, wherein thealkylating agent/3-alkoxy-4-hydroxybenzaldehyde molar ratio rangesbetween 1.0 and 2.0.
 20. Process according to claim 19, wherein thetemperature of the O-methylation reaction is between 60° C. and 100° C.21. Process according to claim 20, characterized in that, firstly, the3-alkoxy-4-hydroxybenzaldehyde and the basic solution are introduced andthe mixture is then brought to the reaction temperature, in that thealkylating agent is added and optionally the base, if required, and inthat an organic phase comprising the 3-alkoxy-4-methoxybenzaldehyde isrecovered and is then separated out.
 22. Process according to claim 21,characterized in that the two reaction steps are performed under anatmosphere of inert gas.
 23. Process according to claim 1 wherein thealkyl or cycloalkyl radical is ethyl, propyl, isopropyl, n-butyl,isobutyl, sec-butyl or tert-butyl.
 24. Process according to claim 4wherein the strong acid is selected from the group: hydrochloric acid,hydrobromic acid, hydriodic acid, sulphuric acid, pyrosulphuric acid, orperchloric acid, fluorosulphonic acid, chlorosulphonic acid,tri-fluoromethanesulphonic acid, methanesulphonic acid, ethanesulphonicacid, ethanedisulphonic acid, benzenesulphonic acid, benzenedisulphonicacids, toluene sulphonic acids, naphthalenesulphonic acids,naphthalenedisulphonic acids and mixtures thereof.